PROJECT SUMMARY/ABSTRACT: As central mediators of the DNA damage response, the BRCA1 and BRCA2 proteins normally act to preserve genome integrity. Indeed, their genome maintenance functions are thought to be a major, if not the principal, means by which BRCA1/2 suppress tumor formation in normal cells. In particular, BRCA1 and BRCA2 are required for double-strand DNA break repair by homologous recombination (DSBR-HR). However, recent studies have uncovered another BRCA1/2 function that also preserves genome stability and thus potentially contributes to the tumor suppression activity of both proteins. During DNA replication, BRCA1 and BRCA2 have been shown to promote genome integrity by protecting stalled replication forks (SRFs) from nucleolytic degradation. These findings raise critical questions regarding the mechanisms of BRCA1/2 tumor suppression and the prospects for therapeutic targeting of the BRCA1/2 pathway. For example, is the tumor suppression activity of the BRCA1/2 pathway mediated through its ability to promote DSBR-HR or SRF stability, or both? Also, can the DSBR-HR or SRF stability functions of BRCA1/2 be restored for prophylactic and/or therapeutic purposes in women who carry BRCA1/2 mutations? Our preliminary studies have uncovered a novel mechanism by which SRF stability can be reconstituted in BRCA1 mutant cells. In particular, we show that depletion of SMARCAL1, a DNA translocase implicated in replication fork dynamics, restores both SRF stability and chromosome integrity in BRCA1-deficient cells subjected to replication stress. On the basis of these results, we hypothesize that inappropriate remodeling of replication forks by SMARCAL1 in BRCA1- deficient cells can increase genome instability and thereby predispose these cells to breast cancer. To test these hypotheses, we will 1) define the mechanisms by which SMARCAL1 inactivation rescues SRF stability and chromosomal integrity in BRCA1-deficient cells, and 2) determine whether tumor suppression activity can be restored to BRCA1-mutant cells by SMARCAL1 inhibition.